Although the role of ambient air pollution in asthma prevalence has not yet been determined, it seems likely that air pollution is an aggravating factor. It seems unlikely, however, that exposures from hazardous-waste sites could have played a part in the generally increased prevalence of asthma, given the relatively small size of the potentially exposed populations. The role of exposures from hazardous-waste sites in the development of respiratory symptoms cannot be readily evaluated.

A crucial lesson from the recent history of environmental epidemiology has been the critical role played by the general availability of monitoring data for a number of air pollutants. Many of the analyses discussed above could not have been undertaken without this extensive data base on pollutants for which monitoring data are routinely acquired. The nature of the data base also has shaped epidemiologic studies. For example, fewer studies of daily exposure have been done for particulates, because these often are sampled only every sixth day. This has hindered the attempt to replicate the London mortality analyses, except in rare cases, such as Steubenville, Ohio, where sampling results were available on a daily basis (Schwartz and Marcus, 1990) or in the Utah studies (Pope, 1989; Pope et al., in press). Exposure to hydrocarbons in urban air has not been monitored routinely since the 1970s and there has been little work on their direct effects except in studies of the sick building syndrome (Molhave, 1985; Kjaergaard et al., 1989). Routine monitoring of ambient air around hazardous-waste sites is not feasible because of their number, the low likelihood of detection in most cases, and cost. In addition, the small size of the exposed populations in most cases makes the sites difficult to study with standard epidemiologic techniques. Nevertheless, more systematic assessments of where such monitoring and such studies might be appropriate needs to be done early in the process of identifying and describing sites for study. It might well be appropriate that one major site with a nearby exposed population should be intensively studied over a period of a year or so, to acquire data that might be applicable to similar sites.

A variety of methodological approaches have been taken to the study of air pollution epidemiology. These can be applied to the study of hazardous wastes, but are likely to vary as to their success.